Low frequency flow oscillations may arise in air conditioning systems using a transverse fan situated downstream of a plate-fin heat exchanger. These oscillations are associated with swirling flow, counter to the fan rotation, between the downstream face of the heat exchanger and fan inlet. Such conditions cause excessive flow incidence angles over a local sector of the impeller inlet, producing retarded, or stalled, flow within that sector.
The localized nature of the stalled flow leads to its being unstable and oscillatory, with frequency, f.sub.s, in the range of 30 to 80 percent of the fan rotational frequency, n. Blade interaction with the unsteady, oscillating stall results in excess noise with a frequency corresponding to the product of the stall oscillation frequency, f.sub.s, the number of blades in the impeller, Z, and the fan rotational frequency, n. The product of Z.n is the blade passing frequency, BPF, and the excess noise is, therefore, sub-BPF noise with frequency in the range of 30 to 80 percent of BPF.